1. Field of the Invention
The present invention concerns a method, a magnetic resonance apparatus and a computer program to generate images by means of a parallel acquisition technique.
2. Description of the Prior Art
Parallel acquisition techniques (PAT) enable the spatial frequency space (known as k-space) to be undersampled during a data acquisition (a measurement) in magnetic resonance tomography (MRT), i.e. to fall below the density of the measured data points or data lines that is required according to the Nyquist theorem and to approximately calculate the missing data points (most often whole data lines) during the image reconstruction or to directly suppress the aliasing artifacts resulting from the undersampling in the associated image space. The measurement time that must be applied to acquire the raw data can thus be markedly reduced. Furthermore, typical artifacts that occur given special applications or sequence techniques can be reduced in part with the aid of the parallel acquisition techniques. The requirement in order to be able to apply parallel acquisition techniques is multiple acquisition coils and the knowledge of the spatial sensitivities of the acquisition coils used in the acquisition of the raw data; this is also called a knowledge of the coil sensitivities. The coil sensitivities can be calculated approximately from what are known as coil calibration data. The coil calibration data are normally additionally measured. The spatial information of the measurement data that is missing due to the undersampling is then compensated with the aid of the coil calibration data or with the use of the coil sensitivities of the acquisition coils that are calculated from the coil calibration data. The missing data points are thereby either substituted with the use of the coil calibration data (or with the use of the coil sensitivities of the acquisition coils that are calculated from the coil calibration data) and the measured data points (measurement data), or the aliasing artifacts resulting from the undersampling are directly suppressed in image space with the use of the coil sensitivities. In both cases this is called a PAT reconstruction.
The coil sensitivities of the acquisition coils depend on, among other things, the orientation of the acquisition coils on the examination subject (for example a patient) and the respective load in the field, thus on properties of the examination subject at the location of the acquisition coil. The coil sensitivities therefore must be re-determined at least for every examination subject. The sensitivity of the acquisition coils can also be affected during a measurement by movements (in particular macroscopic movements) of the examination subject, for example breathing movements or other movements of a patient. The coil calibration data are therefore ideally reacquired for every measurement and in close temporal proximity to the measurement data.
Different methods to measure the coil calibration data or, respectively, to determine the coil sensitivities are described in the article by M. Griswold et al., “Autocalibrated coil sensitivity estimation for parallel imaging”, M.R Biomed. 2006; 19: 316-324, for example.
However, the known methods to determine coil calibration data are not suitable for every sequence technique for the data acquisition and/or are undesirably time-consuming.